In a dense wavelength division multiplexed (DWDM) optical network, an aggregate optical signal travelling on a single optical fiber occupies multiple wavelengths, each of which carries a respective high-speed optical data signal. Although they may travel together along multiple spans throughout the network, the individual optical data signals carry information that is independent and is associated with its own source and destination network elements.
It is often desirable for a downstream network element in a DWDM network to have knowledge of the average intensity of the optical data signal occupying each wavelength. This is to allow the network element to initiate control functions such as amplifier gain adjustment and protection switching in a timely fashion. In order to keep costs to a minimum, it is desirable for the downstream network element to have access to the intensity information at multiple wavelengths without having to resort to wavelength demultiplexing and conversion of multiple (possibly hundreds of) optical signals into electrical form.
A solution to this problem for low density wavelength division multiplexed optical networks is described in U.S. Pat. No. 5,513,029 to Roberts et al., issued on Apr. 30, 1996, assigned to the assignee of the present invention and hereby incorporated by reference herein in its entirety.
In accordance with U.S. Pat. No. 5,513,029, a transmitter first modulates the optical data signal on each wavelength with a respective dither signal of a certain amplitude. The dither signals are described as being pseudo-random noise (PN) sequences which are distinct for each wavelength. The transmitter then measures the modulation depth of each individual resultant signal after modulation with the corresponding dither signal and adjusts the amplitude of the dither signal until a known “modulation depth” is achieved. (The “modulation depth” is commonly defined as the root-mean-square (RMS) value of the dither signal divided by the average intensity of the optical data signal being modulated.)
Thus, a receiver located at a downstream network element can estimate the average intensity of the optical data signal at multiple wavelengths of an incoming optically multiplexed signal by: (a) tapping a portion of the multiplexed signal using an inexpensive optical coupler; (b) converting the tapped optical signal to electrical form; (c) low-pass filtering the electrical signal to provide an aggregate dither signal; (d) separating the dither signals by matching them with known PN sequences; (e) measuring the RMS value of each dither signal; and (f) dividing the RMS value of each dither signal by the known modulation depth.
Unfortunately, the system disclosed in U.S. Pat. No. 5,513,029 has several drawbacks, which either cause the system to have poor performance or make the system prohibitively complex to implement, or both. This is especially true in the context of a dense WDM network, where there may be hundreds of optical carriers sharing a relatively narrow portion of the electromagnetic spectrum.
For example, in order to achieve good performance at the receiver, the RMS value of each dither signal present in the aggregate dither signal must be determined to a high degree of accuracy. Of course, this can only occur if the receiver accurately detects the presence of each dither signal in the aggregate dither signal. If the individual dither signals are PN sequences (as per U.S. Pat. No. 5,513,029), then in order to achieve the requisite degree of accuracy, the receiver must sample the aggregate dither signal at the center of the “chip period” of each PN sequence.
However, because the dither signals are generated on an independent basis, possibly by sources located at different parts of the network, there will generally be phase offsets among the dither signals contained in the aggregate dither signal. Therefore, in order to compensate for such differences in phase, the sampling rate used at the receiver must be much higher than the chip rate. It is therefore apparent that there exists a trade-off between sampling precision (leading to accurate RMS estimates and hence accurate estimates of average intensity) and computational simplicity.
The problem is no less severe if the dither signals used for different optical wavelengths occupy different frequencies rather than different PN sequences. In this case, the step of distinguishing the various dither signals would involve passing the aggregate dither signal through a parallel array of filters, one for each wavelength. As the number of wavelengths (and filters) increases, the bandwidth of each filter will have to be decreased in order to allow only the desired dither signal to pass through to the RMS detection stage, with the effect of deleteriously increasing the filter design complexity.
Moreover, it has been observed that framing of an optical data signal at 8 kHz causes the appearance of spurious frequency lines at multiples of 8 kHz in the frequency spectrum of the optical data signal. If such lines fall within the bandwidth of one or more of the filters, then it is apparent that the RMS value estimated as a function of each such filter's output will be biased. In a more general case, e.g., where the optical data signal is asynchronous, it may be impossible to predict the location of spurious frequency contamination, with the end result being the production of randomly biased RMS estimates.
Clearly, it would be a huge advantage to provide a system which is capable of estimating the average intensity of an optical signal at each of one or more wavelengths with reasonable accuracy and low computational complexity.